DE60124368T2 - Soft magnetic material of Fe-Cr alloy and process for its production - Google Patents

Description

The The present invention relates to a soft magnetic material which as a nucleus, a yoke or the like, in different ways from magnetic sensors, such as power steering, fuel injection systems for motor vehicles and magnetic AC circuits, such as solenoid valves, installed were, is usable.

One AC magnetic circuit becomes an electromagnetic Induction sensor, for example, a differential magnetic coil sensor or a flow sensor or a mechanical quantity sensor, for example a magnetostrictive Torque sensor or a phase-differentiated torque sensor, built-in. Another type of sensor that uses an exciter coil as one Detection coil used is already known. A core and a Yoke, as parts of such a magnetic AC circuit, are made of soft magnetic material, such as pure iron, Si-steel, soft ferrite or mu-metal.

The Displacement of an object or a torque is considered one small change detected in the impedance or voltage of the detection coil, when moving the object by applying alternating current arises to the excitation coil, so that an alternating field is generated.

With The development of magnetic sensors is increasing the demands for improvement the measuring accuracy more and more. Because the reduction of noise while the detection of an output signal voltage for improvement the measurement accuracy is inevitable, will necessarily be electric high-frequency current (for example 100 Hz-5 kHz) applied with a sine wave or square wave to an excitation coil.

however elevated the eddy current loss of electromagnetic soft iron (SUYP), that usually was used as a soft magnetic material, proportional to Frequency increase of the applied magnetic field, resulting in a decrease magnetic induction leads, the for the sufficient voltage is necessary. Si steel is advantageous in lower eddy current loss, due to its high specific electrical resistance, compared with electromagnetic Soft iron, however, increased the Si content necessarily, the reduction of magnetic Induction in an alternating field with the frequency of not less than 1 kHz. Although an increase of the Si content, the specific electrical resistance is effective elevated, Si-steel is hardened and the press processability deteriorates.

The corrosion resistance is also one of the required properties of soft magnetic material, which is expected to be used in a specific environment becomes. However, electromagnetic soft iron and Si steel are poor in the corrosion resistance. corrosion resistance can by formation of a Ni or Chromatbehandlungsschicht however, such plating causes a cost increase Product. The plating unfavorably builds magnetic properties off and changed due to the irregularity in the thickness of the plating layer also has magnetic properties.

Mu-metal, in particular Mu-metal C or Permalloy C, is a material that in the magnetic AC characteristic with high specificity electrical resistance excellent, but very expensive is. Soft ferrite has a high electrical resistivity with less reduction in magnetic induction in a high frequency range of not less than 10 kHz, compared with metal material, however is in contrast its magnetic flux density in the frequency domain of not more than 5 kHz lower than that of the metal material.

JP-A-08 047 235 discloses a ferritic Fe-Cr alloy, which in a Temperature between 700 and 1200 ° C heat treated becomes.

Fe-Cr alloy has been used as a yoke for a stepper motor, due to its high specific electrical Resistance, good corrosion resistance and cost-effectiveness, compared with Mu metal. If, however, usual Fe-Cr alloy as part a magnetic circuit, such as a magnetic sensor, in one weak magnetic field of less than 10 Oe, with a frequency of 100 Hz-5 kHz, works, is not enough output, that for An accurate measurement is necessary when scoring at a detection terminal.

The The present invention seeks to provide a new, low cost Soft magnetic Fe-Cr material, the excellent properties as a magnetic sensor operating in a weak high-frequency magnetic field works, as well as corrosion resistance having.

The proposed new soft magnetic Fe-Cr material has a specific electric resistance of not less than 50 μΩ. cm and a metallurgical structure composed of ferritic grains at an area ratio of not less than 95%, with precipitates of 1 μm or less in particle size at a ratio of not more than 6 × 10 ⁵ / mm ² in number ,

The soft magnetic Fe-Cr material has the composition consisting of C up to 0.05 mass%, N up to 0.05 mass%, Si up to 3.0 mass%, Mn up to 1.0 mass%, Ni up to 1.0 mass%, P up to 0.04 mass%, S up to 0.01 mass%, 5.0-20.0 mass% Cr, Al up to 4.0 mass%, 0-3 mass % Mo, 0-0.5 mass% Ti, and wherein the balance Fe is excluding unavoidable impurities under conditions of (1) and (2). 4.3 ×% Cr + 19.1 ×% Si + 15.1 ×% Al + 2.5 ×% Mo ≧ 40.2 (1) 64 ×% Si + 35 ×% Cr + 480 ×% Ti + 25 ×% Mo + 490 ×% Al ≧ 221 ×% C + 247 ×% N + 40 ×% Mn + 80 ×% Ni + 460 (2)

The soft magnetic material is prepared by providing an Fe-Cr alloy with the specified composition, forming the Fe-Cr alloy into an objective shape, and heat-treating the Fe-Cr alloy formed in a zone between 900 ° C and a temperature T. (° C) defined by formula (3), in a vacuum or reducing atmosphere. The phrase "soft magnetic material" means a material that has not yet been shaped into a magnetic part, in various forms of sheets, rods or wires, in response to its application. T (° C) = (64 ×% Si + 35 ×% Cr + 480 ×% Ti + 490 ×% Al + 25 ×% Mo + 480) - (221 ×% C + 247 ×% N + 40 ×% Mn + 80 ×% Ni) (3)

1 Fig. 12 is a schematic view for explaining a detection circuit of a magnetostrictive torque sensor.

2 Fig. 12 is another schematic view for explaining a detection coil installed in the detection circuit.

3 FIG. 12 is a graph showing an effect of the specific electric resistance on the magnetic induction of a soft magnetic Fe-Cr material.

4 Fig. 10 is a graph showing an effect of the content of martensite grains on the magnetic induction of a soft magnetic Fe-Cr material.

5 Fig. 10 is a graph showing an effect of a plurality of fine precipitates on the magnetic induction of soft magnetic Fe-Cr material.

If a soft magnetic material loaded with a magnetic alternating field becomes, energy losses occur in the soft magnetic material.

hysteresis loss which represents one of the energy losses is due to the interaction between the ferromagnetic domain walls and precipitates or Lattice defects from the suppression of Movement derived from ferromagnetic domain walls. In this Meaning, the hysteresis loss is reduced as the precipitates and increase lattice defects. Regarding Fe-Cr alloy it is practically important to produce fine precipitates and martensite grains to inhibit.

Eddy current loss is also one of the detrimental energies. The eddy current; i.e. one Secondary current, the by a change the magnetic intensity due to conductivity of the soft magnetic metal material is induced an energy loss caused by ohmic loss. To reduce the eddy current loss, the specific electrical Resistance of the soft magnetic material necessarily be greater to dampen the eddy current or to prevent.

Regarding In these aspects, the inventors have the effects of the specific electrical resistance and a metallurgical structure as well the state of precipitates on the order of hysteresis and Eddy current losses studied and tested, and also mechanisms of high magnetic flux density in a weak magnetic field alternating field examined. Although a common one Soft-magnetic Fe-Cr material must always be at a temperature above his solid solution line (i.e., a boundary between a solid solution and a mixed phase) to the resolution must be heated by fine carbide particles in its matrix, heating to too high a temperature causes generation of γ phase, the while of cooling is transferred to martensite grains. Therefore, it is necessary to determine the excretions that are on soft magnetic properties exert harmful influences, and also conditions of composition and heat treatment, without generation a martensite phase harmful Determine excretions in a matrix.

A magnetostrictive torque sensor, one of the magnetic sensors, has an in 1 shown detection circle. A rotating shaft 1 is held at a position leading to an excitation coil 2 and a detection coil 3 has. The detection coil 3 has a magnetic circuit with a soft magnetic part 5 is equipped on a supply wire 4 is wrapped, as in 2 shown. When a predetermined voltage V is applied between the terminals to generate an electric circuit i, a magnetic flux line Φ becomes between the soft magnetic portion 5 and a measurement object S generated. A change in the magnetostriction caused by stress due to a torque is transmitted through the detection coil 3 as a change of the output signal voltage induced by the magnetic flux Φ which is transmitted through the exciting coil 2 is generated by the oscillator 6 and power amplifier 7 is controlled, recorded. A result of the detection is provided by a synchronous detector 83 and an amplifier 9 output.

One soft magnetic part, such as a core, in the detection circuit is incorporated by mechanical processing of a soft magnetic Made steel sheet or the like to a predetermined shape. The thus processed soft magnetic material is due to the residual stresses or mechanical residual stresses caused by the mechanical processing introduced become poor in magnetic permeability, which is bad magnetic induction results. Such harmful influences of mechanical stresses are released by heat treatment eliminated.

The Inventors have the effects of various factors on the magnetic induction from a soft magnetic part as below investigated: magnetically soft Fe-Cr steels, which differ from each other in the distinguish electrical resistance become mechanically one Circular shape processed, heat treated under different conditions and then provided for measuring the magnetic flux density. The magnetic flux density is using a B-H analyzer in an exciting weak magnetic field with an oscillation frequency of 1 kHz and magnetic intensity of 1 Oe measured.

The measurement results are in 3 shown. It is noted that a soft magnetic material in the magnetic induction at a specific electrical resistance greater than 50 μΩ. cm is greatly improved. The inventors further have the effects of the compositions of soft magnetic materials whose electrical resistivity is greater than 50 μΩ. cm is examined for electrical resistivity, and found that the specific electrical resistance ρ of the Fe-Cr alloy is defined by the formula mentioned below. Consequently, the above-mentioned formula (1) is determined to have the electrical resistivity ρ greater than 50 μΩ. cm to achieve. ρ (μΩ · cm) = 4.3% Cr + 19.1% Si + 15.1% Al + 2.5% Mo + 9.8

however have soft magnetic parts made of the same Fe-Cr alloy be produced, the feature that magnetic induction in response to the heat treatment conditions for one Use in a magnetic circuit that works in a weak magnetic field from 1 Oe or similar works, deviates substantially. The inventors have the effects from metallurgical structures to magnetic induction to Finding out the causes leading to the deviation from magnetic induction to lead, by observing the metallurgical structure of a heat treated soft magnetic material examined. As a result, the inventors have found that the metallurgical structure, the martensite grains or fine precipitates in a ferritic single phase free of martensite grains which is very poor in magnetic induction (i.e. has bad sensor characteristic), even if the soft magnetic Part made of the same Fe-Cr alloy will be produced.

The unfavorable effect of martensite grains on magnetic induction becomes in the Fe-Cr alloy tion, containing martensite grains at a level of 5% by volume or more, is clearly Precipitations of 1 μm or larger in particle size have substantially no effect on magnetic induction, but magnetic induction is affected by fine precipitates of less than 1 μm in particle size. The magnetic induction deteriorates as the number of precipitates increases. In particular, a distribution of fine precipitates of less than 1 μm, with a ratio of 6 × 10 ⁵ / mm ² in number, causes a marked decrease in magnetic induction, as in 5 shown.

These results prove that an Fe-Cr alloy usable as a soft magnetic part incorporated in a magnetic circuit such as a magnetic sensor operating in a high-frequency excitation field has a resistivity not less than 50 μΩ. cm, and such a heat-treated metallurgical structure including martensite grains of not more than 5% by volume with precipitates of 1 μm or less in particle size at a ratio of not more than 6 × 10 ⁵ / mm ² .

Fine Excretions of 1 μm or less in the particle size can through Heating a Fe-Cr alloy to a temperature higher than 900 ° C strong be reduced. The effect of heat treatment on the decrease of fine precipitates is by soaking the Fe-Cr alloy, preferably for 30 Minutes or longer, clearly observed. However, too high a soak temperature means overheating the Fe-Cr alloy in a γ-zone, the while of cooling for the production of martensite grains leads.

A such kind of steel as the γ-phase caused at a heating temperature below 900 ° C, can not a metallurgical structure to be transformed from a ferrite Single phase is composed, which is to improve magnetic Induction in suppression of fine excretions is effective. Taking into account the practical Accuracy of temperature control in a standard oven is supposed to be a temperature range for heat treatment for the Generation of a ferrite single matrix, the less fine precipitates contains, without martensite grains a tolerance of at least ± 20 ° C (ideally ± 50 ° C), with respect to one have predetermined temperature.

A Initial temperature T (° C) for generating a γ-phase is characterized by the above Formula (3) according to the investigations the inventor reported on the effects of alloying elements. On the other hand, the initial temperature T should be considered the accuracy of temperature control in a conventional Oven at a tolerance of at least ± 20 ° C not lower than 900 ° C, to the production of both martensite grains and fine precipitates to inhibit.

Therefore is the initial temperature T (° C) at a temperature not lower than 940 ° C. The above-mentioned formula (2) is obtained by substituting formula (3) into the relationship of T ≧ 940 ° C. Farther will be a temperature for the heat treatment preferably at 940 ° C or higher adjusted to improve the growth of crystal grains Magnetic property without producing martensite phase to promote. A Ideal temperature T is at least 980 ° C.

The Generation of a metallurgical structure consisting of a ferrite Single phase is to increase an initial temperature T by adding a ferrite-stabilizing element / elements, like Si, promoted to a Fe-Cr alloy. However, one causes too high an addition of ferrite stabilizing element (s) a decrease in the rollability and press workability as well as the appearance of surface defects.

The reduction of martensite grains to a level of not more than 5% by volume effectively suppresses the decrease of magnetic induction as in 4 shown. The reduction of martensite grains is made by increasing the difference between a ferritizing intensity (represented by 11.5 ×% Si + 11.5 ×% Cr + 49 ×% Ti + 12 ×% Mo + 52 ×% Al) and an austenitizing intensity ( reproduced by 420 ×% C + 470 ×% N + 7 ×% Mn + 23 ×% Ni). Such a difference of more than 124 allows absolute suppression of the generation of martensite grains since an Fe-Cr alloy can be heated up to about 1030 ° C without generation of γ-phase.

The initial temperature T for generating the γ-phase is higher than an increase in difference between the ferritizing and austenitizing intensities to promote the production of a ferritic single-phase metallurgical structure. However, the increase in the difference requires a plurality of ferritizing elements to be added to the Fe-Cr alloy, resulting in an Ab construction on rollability and Druckverarbeitbarkeit and occurrence of surface defects leads. As a result, the composition of the proposed new Fe-Cr alloy is preferably determined as follows:

C up to 0.05% by weight

C is a for the magnetic property of a soft magnetic Fe-Cr material harmful element, since it is the production of Martensitkömern and excretion of carbides accelerated. The Fe-Cr alloy becomes harder when the C content elevated, which results in poor print processability. These harmful influences are controlled by controlling the C content to not more than 0.05% by mass suppressed.

N up to 0.05% by weight

N is also a harmful Element, as it accelerates the production of martensite grains and due the increase the hardness deteriorates the press workability of the Fe-Cr alloy. In this sense, an upper limit of the N-content is set at 0.05% by mass or adjusted.

Si up to 3.0% by weight

Si is an alloying element used for the increase of electrical resistivity and magnetic induction is effective in a magnetic alternating field. The additive Si preferably suppresses the production of martensite, causing harmful influences transmits the soft magnetic property. However caused too much addition from Si an increase of hardness and a decrease in print processability. In this sense will set an upper limit of Si content at 3.0 mass%.

Mn up to 1.0% by weight

Mn is an impurity element that melts into an Fe-Cr alloy melt from a raw material such as scrap in an alloy melting step and accelerates the production of martensite. Therefore, an upper limit of Mn content is set at 1.0 mass%.

Ni up to 1.0% by weight

Ni is also an impurity element that melts into an Fe-Cr alloy melt from raw material, such as scrap, in an alloy melting step and accelerates the production of martensite. Therefore, an upper limit of the Ni content is set at 1.0 mass%.

P up to 0.04 mass%

P is included as phosphides, the harmful influences exert the soft magnetic property; therefore, an upper Limit of the P-content at 0.04 mass%.

S up to 0.01 mass%

S is included as sulfides, causing harmful influences the soft magnetic property exerts; therefore, it becomes an upper limit of the S content at 0.01% by mass.

5.0-20.0% by weight Cr

Cr is an alloying element that suppresses the production of martensite increased electrical resistivity of an Fe-Cr alloy, the magnetic induction in a magnetic alternating field as well as Si improved and also the corrosion resistance improved. These effects become more at a Cr content as 5.0 mass% (preferably 10 mass%) is clearly observed. however reduces too high an addition of Cr above 20.0 mass% due to the increase the hardness the magnetic induction and print processability of the Fe-Cr alloy.

Al up to 4.0% by weight

Al is an alloying element that has the electrical resistivity and the magnetic In production in a magnetic alternating field, just like Si and Cr, greatly increased. However, too much addition of Al causes occurrence of surface defects originated from type A ₁ inclusions, so that an upper limit of the Al content is set at 4.0 mass%.

0-3 mass% Mo

Not a word is an optional alloying element that produces the same as Cr suppressed by martensite, increases the electrical resistance, improved magnetic induction in a magnetic alternating field and also the corrosion resistance improved. Excessive addition of Mo above 3% by mass cures an Fe-Cr alloy significantly and reduces their Druckverarbeitbarkeit.

0-0.5 mass% Ti

Ti is an optional alloying element which, like Cr and Mo, is the Production of martensite suppressed, however, the appearance of surface defects caused by titany inclusions. In this sense For example, an upper limit of the Ti content at 0.5 mass% is determined.

EXAMPLE

• A Wert A: 4,3 × % Cr + 99,1 × % Si + 15,1 × % Al + 2,5 × % Mo
• A Wert B:(64 × % Si + 35 × % Cr + 480 × % Ti + 490 × % Al + 25 × % Mo) – (221 × % C + 40 × % Mn + 80 × % Ni + 247 × % N) Die unterstrichenen Zahlen liegen außerhalb des Umfangs der vorliegenden Erfindung.

Various Fe-Cr alloys having compositions shown in Table 7 were melted in a 30 kg high-frequency furnace in a vacuum atmosphere. A soft magnetic Fe-Cr alloy sheet of 2.0 mm thick was prepared from each alloy by casting, forging, hot rolling, cold rolling, finish heat treatment and then descaling. TABLE 1: Fe-Cr alloys used in Example 1

• A value A: 4.3 ×% Cr + 99.1 ×% Si + 15.1 ×% Al + 2.5 ×% Mo
• A value B: (64 ×% Si + 35 ×% Cr + 480 ×% Ti + 490 ×% Al + 25 ×% Mo) - (221 ×% C + 40 ×% Mn + 80 ×% Ni + 247 ×% N) The underlined numbers are beyond the scope of the present invention.

Test pieces were cut from each soft magnetic Fe-Cr alloy sheet.

After this a ring-shaped test piece of 45 mm in outside diameter and 33 mm in inner diameter under conditions shown in Table 2 heat treated became its magnetic flux density B with a B-H analyzer measured in a magnetic field of 1 Oe at a frequency of 1 kHz.

Another test piece measuring 30 mm x 30 mm was etched in a flux / nitric acid-glycerol solution (HF: HNO ₃ : glycerol = 2: 1: 2) and then a dot counting method using an optical microscope for measurement subjected to martensite.

The same test piece was etched by SPEED (Selective Potentiostatic Etching by Electrolytic Dissolution) method and then observed by a scanning electron microscope. The number of fine precipitates of 1 μm or less in particle size, shown on a monitor screen, was counted to reduce the number of fine precipitates gene per 1 mm ² to calculate. Further, a test piece of 5 mm in width and 150 mm in length was subjected to the Wheatstone Bridge method for measuring its specific electrical resistance.

on the other hand the soft magnetic Fe-Cr alloy sheet has become cores for excitation and detection coils are pressure processed, and then under the same Conditions such as the ring magnet heat treated. The nuclei were examined to determine the presence or absence of cracks. The print processability of the Fe-Cr alloy sheet was evaluated in response to the occurrence of cracks.

Each core was tested in a magnetostrictive torque sensor (shown in 1 ) built-in. An output signal voltage of a detection coil was measured in accordance with an input torque in a magnetic field of 1 Oe at an oscillation frequency of 1 kHz applied to an excitation coil. The measurement voltage was compared with a standard value (100) representing an output signal voltage required for a sensor, and the sensor characteristic became less than 100 as good (O), slightly deficient (Δ) at a value of 100-80 or rated as deficient (X) at a value of less than 80.

The Test results are taken together with the heat treatment conditions in Table 2 is shown.

The Results show that the test pieces numbers 1-9, whose specific electrical resistance, proportion of martensite and number were controlled according to the invention on fine precipitates, a magnetic flux density of not less than 500 G and a higher output signal voltage produced. Therefore, the Fe-Cr alloy sheets Numbers 1-9 as nuclei for a torque sensor that improves in the sensor characteristic is, usable.

On the other hand, the Fe-Cr alloy sheet No. B1 had a magnetic induction which is excessively distributed in number because of its metallurgical structure in which fine precipitates of 1 μm or less in particle size are excessively distributed at a ratio above 6 × 10 ⁵ / mm ² , is significantly deteriorated. (In the result, a core made of the alloy sheet No. B1 deteriorated in the sensory property.

• T = (64 × %Si + 35 × % Cr + 480 × % Ti + 490 × % Al + 25 × %Mo + 480) – (221 × % C + 40 × % Mn + 80 × % Ni + 247 × % N)
• O: gut, Δ: etwas mangelhaft, X: mangelhaft

The test piece No. 13 made of an Fe-Cr alloy sheet having the same composition but heat-treated at a low temperature in a magnetic field had a magnetic induction which, due to its metallurgical structure, had excessively fine precipitates of 1 μm or less in particle size was significantly deteriorated. A core of the alloy sheet No. 13 was also deteriorated due to the decrease of the magnetic induction in the sensor property. The test piece No. 14, which has been heat-treated at too high a temperature, on the contrary, contains an amount of martensite grains in a heat-treated state. Therefore, the core made of the alloy sheet No. 14 had a magnetic induction that was significantly deteriorated due to the generation of martensite, resulting in poor sensor performance. TABLE 2 Effects of Heat Treatment Conditions, Electrical Resistivity, and Metallurgical Structure on Press Workability, Magnet Property, and Sensor Property

• T = (64 ×% Si + 35 ×% Cr + 480 ×% Ti + 490 ×% Al + 25 ×% Mo + 480) - (221 ×% C + 40 ×% Mn + 80 ×% Ni + 247 ×% N)
• O: good, Δ: a bit deficient, X: deficient

The underlined numbers are outside the scope of the present Invention.

The as mentioned above Soft magnetic according to the invention Material is made of an Fe-Cr alloy with an electrical resistance of not less than 50 μΩ. cm and a metallurgical structure containing few martensite grains, and suppresses the distribution of fine precipitates produced. by virtue of high specific electrical resistance and specific metallurgical structure produces the soft magnetic material strong magnetic induction, resulting in excellent sensor performance, even in a weak magnetic field that excites at high frequency has been. As a result, a sensor having good measurement accuracy becomes by incorporating the soft magnetic material as a core or yoke in a magnetic circuit, such as an electromagnetic induction sensor or a mechanical quantity sensor.

Claims (2)

A soft magnetic Fe-Cr material having an electrical resistance of not less than 50 μΩ cm and a metallurgical structure composed of ferritic grains having an aspect ratio of not less than 95%, with precipitates of 1 μm or less in particle size at a ratio of not more than 6 × 10 ⁵ / mm ² in number, and which of C up to 0.05 mass%, N up to 0.05 mass%, Si up to 3.0 mass%, Mn up to 1.0 mass%, Ni up to 1.0 mass%, P up to 0.04 mass%, S up to 0.01 mass%, 5.0-20.0 mass% Cr, Al is up to 4.0% by mass, 0-3% by mass Mo, 0-0.5% by mass of Ti, the balance being Fe excluding unavoidable impurities under conditions of formulas (1) and (2). 4.3 ×% Cr + 19.1 ×% Si + 15.1 ×% Al + 2.5 ×% Mo ≧ 40.2 (1) 64 ×% Si + 35 ×% Cr + 480 ×% Ti + 25 ×% Mo + 490 ×% Al ≧ 221 ×% C + 247 ×% N + 40 ×% Mn + 80 ×% Ni + 460 (2) A process for producing a soft magnetic Fe-Cr material comprising the steps of: providing an Fe-Cr alloy having a composition consisting of C up to 0.05% by mass, N up to 0.05% by mass, Si up to 3.0 mass%, Mn up to 1.0 mass%, Ni up to 1.0 mass%, P up to 0.04 mass, S up to 0.01 mass%, 5.0- 20.0% by mass of Cr, Al up to 4.0% by mass, 0-3% by mass Mo and 0-0.5% by mass of Ti, the remainder being Fe excluding unavoidable impurities, under conditions of the formulas ( 1 and 2) 4.3 ×% Cr + 19.1 ×% Si + 16.1 ×% Al + 2.5 ×% Mo ≥ 40.2 (1) 64 ×% Si + 35 ×% Cr + 480 ×% Ti + 25 ×% Mo + 490 ×% Al ≧ 221 ×% C + 247 ×% N + 40 ×% Mn + 80 ×% Ni + 460 (2) forming the Fe-Cr alloy into an objective form and heat-treating the Fe-Cr alloy formed in a zone between 900 ° C and a temperature defined by the formula (3) in a vacuum or reducing atmosphere. T (° C) = (64 ×% Si + 35 ×% Cr + 480 ×% Ti + 490 ×% Al + 25 ×% Mo + 480) - (221 ×% C + 247 ×% N + 40 ×% Mn + 80 ×% Ni) (3)